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Membrane Processes Membrane Processes –– Part 2Part 2

CE 370

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Reverse osmosis Reverse osmosis -- DefenitionDefenitionIt is the process of forcing a solvent (like water) from a region of high solute (such as salts of Soudium, Potasium, etc…) concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure.It is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied. The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute.

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Reverse osmosis Reverse osmosis –– How it worksHow it works

Reverse osmosis occurs when the water is moved across the membrane against the concentration gradient, from lower concentration to higher concentration. To illustrate, imagine a semipermeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution. In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the fresh water side.

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Reverse osmosis Reverse osmosis –– How it worksHow it works

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Reverse osmosis Reverse osmosis –– Osmotic PressureOsmotic Pressure

RTVnφνπ =

The osmotic pressure of solutions of electrolytes may be determined by the following equation:

Where;• π = osmotic pressure• φ = osmotic coefficient• ν = number of ions formed from one molecule of electrolyte• n = number of moles of electrolyte• V = volume of solvent• R = universal gas constant• T = absolute temperature

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Reverse osmosis Reverse osmosis –– Osmotic PressureOsmotic Pressure

The osmotic pressure for see water, which has 35,000 mg/l dissolved solids, is 397 psi (2740 kPa) at 25oC.

It can be assumed that an increase of 1000 mg/l salt concentration results in an increase of approximately 11.3 psi (78 kPa) in osmotic pressure.

For example, the osmotic pressure of a solution that has 23,000 mg/l of TDS is:

psi 2601000

3.11000,23 =×=π

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Reverse osmosis Reverse osmosis –– FluxFluxThe main design parameter for a reverse osmosis unit are the production per unit area of membrane and water quality.

The production is measured by the flux of water through the membrane (gal/day-ft2 or l/day-m2).

The flux is related to the pressure by the following equation:

WhereFw = water flux (gal/day-ft2 or l/day-m2))K = mass transfer coefficient (gal/d-ft2-psi or l/d-m2-kPa)∆p = pressure difference between feed and product water (psi or kPa)∆π = osmotic pressure difference between feed and product water (psi

or kPa)

)( π∆−∆= pKFw

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Reverse osmosis Reverse osmosis –– FluxFluxThe membrane flux value furnished by manufacturer is usually for 25oC. Temperature variations causes the flux to vary. Therefore, membrane area correction (AT/A25) should be considered as the following:•• For 10For 10ooC,1.58; 15C,1.58; 15ooC, 1.34; 20C, 1.34; 20ooC, 1.15; 25C, 1.15; 25ooC, 1.00; 30C, 1.00; 30ooC, 0.84C, 0.84

The term (AT/A25) is the ratio of the areas required for temperatures of ToC and 25oC.

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Reverse osmosis Reverse osmosis –– FluxFluxThe flux value will gradually decrease during the lifetime of a membrane which occurs in all membranes and is permanent.

The membrane must be replaced when the flux has reached the minimum acceptable level set by the membrane manufacturers.

Usually, the life span of a membrane is from few months to several years.

Pre-treatment of the raw water is essential to make sure that the membrane serves its design lifetime.

Pre-treatment includes removal of turbidity and other impurities, adjusting the pH, removal of chlorine for some types of membrane materials, adjusting temperature, etc…

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Types of Reverse Osmosis MembranesTypes of Reverse Osmosis MembranesThe most common membrane materials are polyamide thin film composites (TFC) or cellulose-type membranes. TFC membranes are:• More costly, but have greater strength and durability than cellulose-types• They have higher total dissolved solids (TDS) rejection rates • Are more resistant to microbial attack and are more tolerant of high pH• Deteriorate in chlorinated water.

Cellulose type membranes are:• less costly than TCF membranes• Can tolerate chlorine which is commonly used for disinfection of drinking water.

Another type of membrane is a sulfonated polysulfone (SPS) membrane. • tolerant of chlorine and can withstand higher pH levels• are more costly than cellulose-types and less effective than TFC membranes • SPS membranes can be used in RO systems when the water is soft and pH is high

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Types of Reverse Osmosis MembranesTypes of Reverse Osmosis MembranesSpiral wound (like a rolled up newspaper):

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Types of Reverse Osmosis MembranesTypes of Reverse Osmosis MembranesHollow fibers: can be bundled together. This provides a very large surface area for water treatment within a compact tube element.

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Types of Reverse Osmosis MembranesTypes of Reverse Osmosis MembranesHollow fibers.

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Reverse Osmosis Reverse Osmosis -- ExampleExample

Example 14.2 page 406:

A reverse osmosis unit is to demineralize 760,000 l/d of tertiary treated effluent. Pertinent data are as follows: mass transfer coefficient = 0.2068 l/(d-m2)(kPa) at 25oC, pressure between the feed and product water = 2400 kPa, osmotic pressure difference between the feed and product water = 310 kPa, lowest operating temperature = 10oC, and A10C = 1.58 A25C. Determine the membrane area required.

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Reverse Osmosis Reverse Osmosis -- ExampleExampleSolution:

The water flux is given by equation (14.6):

Fw = [0.2068 l/(d-m2)(kPa)]×(2400 kPa – 310 kPa)= 432.21 l/(d-m2) at 25oC

The area is given by:A = (760,000 l/d)/(432.21 l/(d-m2) = 1758.4 m2 at 25oC

The area at 10oC is given by:AT/A25 = 1.58, therefore:

A (at T=10oC) = A25 C × 1.58 = 1758.4 × 1.58 = 2780 m2

)( π∆−∆= pKFw

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Reverse Osmosis Reverse Osmosis -- ApplicationApplicationRO finds extensive applications in the following:• Producing potable water from sea or brackish water.

• Ultrapure water for food processing and electronic industries

• Pharmaceutical grade water

• Water for chemical, pulp & paper industry

• Waste treatment etc.

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Reverse Osmosis Reverse Osmosis -- CostCost

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Membrane InstallationsMembrane Installations

2020

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